Certain types of drill bits are known to be capable of axial cutting or drilling like an ordinary drill bit as well as lateral or side cutting like a milling bit. In order to perform a smooth side cutting action, the cutting edges should be inclined to the axis of the drill bit at a relatively small angle. However, this requires the drill bit to have a relatively large overall diameter such that the central core of the drill bit is sufficiently thick to provide adequate strength, hence the need to use extra material in manufacturing the drill bit and in turn an increase in production cost.
The invention seeks to mitigate or at least alleviate a problem by providing an improved drill bit that has optimum cutting performance and production cost.
According to the invention, there is provided a drill bit capable of axial cutting as well as side cutting, having a generally cylindrical elongate body of a bit diameter, which comprises a frontmost drilling end for axial cutting, a cutting section extending from the drilling end for side cutting, and a shank behind the cutting section. The cutting section is helically cut at a helix angle of substantially 15xc2x0xc2x12xc2x0 to form one pair of diametrically opposite helical flutes whose outermost edges are sharpened to form respective cutting edges, around a solid central core whose diameter is substantially directly proportional to the bit diameter. The length of the cutting edges in the axial direction of the drill bit body is calculated according to the formula: Cutting Edge=Bit Diameterxc3x97xcfx80xc3x97Cc/Tan(Helix Angle), Cc being a constant of a value in the range substantially from 0.85 to 0.90.
Preferably, the constant Cc has a value substantially of 0.85.
In a preferred embodiment, the diameter of the core is calculated according to the formula: Core Diameter=Bit Diameterxc3x97CD, CD being a constant of a value in the range substantially from 0.5400 to 0.60000.
More preferably, the constant CD has a value substantially of 0.5714.
It is preferred that the length of the cutting edges in the axial direction of the drill bit body is substantially directly proportional to the length in that direction of the helical cut forming the flutes.
It is further preferred that the axial length of the cutting edges is calculated based on that of the helical cut according to the formula: Cutting Edge=Helical Cutxc3x97Cc.
It is yet further preferred that the constant Cc has a value substantially of 0.85.
Preferably, each flute has a radial rake angle substantially of 10xc2x0(+3xc2x0/xe2x88x920xc2x0 )
Preferably, the cutting edges have a cutting angle of substantially 45xc2x0xc2x12xc2x0.
In a first preferred embodiment, the bit diameter is substantially 3.5 mm(+0 mm/xe2x88x920.10 mm).
In a second preferred embodiment, the bit diameter is substantially 3.175 mm(+0 mm/xe2x88x920.10 mm).
In a third preferred embodiment, the bit diameter is substantially 6.35 mm(+0 mm/xe2x88x920.10 mm).
It is preferred that the length of the drill bit body is substantially 70 mm to 80 mm.